Why prefer old SA ? Tunnel between Pfsense RC3 doesn't work properly!

I have a Ipsec tunnel between two Pfsense 2.0 RC3 (one is a PC, one is a Alix board).

When the Alix board restart, the tunnel comes up but the PC will prefer old SA and the traffic will not go trough the tunnel… If I disable the option "prefer old SA" on the PC everything is working well, even after reboot the Alix board.